374 



THE FORMS OF CELLS 



[CH. 



(with certain definite relations of height to breadth), then new 

 phenomena may occur. For now, if oil be cautiously withdrawn 

 from the mass by help of a small syringe, the cylinder may be made 

 to flatten down so that its upper and lower surfaces become plane: 

 which is of itself a sufficient indication that the pressure inwards 

 is now nil. But at the very moment when the upper and lower 

 surfaces become plane, it will be found that the sides curve inwards, 

 in the fashion shewn in Fig. 108 B. This figure is a catenoid, which, 



B 



Fig. 108. 



as we have seen, is, like the plane itself, a surface exercising no 

 pressure, and which therefore may coexist with the plane as part 

 of one and the same system. 



We may continue to withdraw more oil from our bubble, drop 

 by drop, and now the upper and lower surfaces dimple down into 

 concave portions of spheres, as the 

 result of the negative internal 

 pressure ; and thereupon the peri- 

 pheral catenoid surface alters its 

 form (perhaps, on this small scale, 

 imperceptibly), and becomes a 

 portion of a nodoid. It represents, 

 in fact, that portion of the nodoid 

 which in Fig. 109 lies between such points as 0, P. While it is easy to 

 draw the outline, or meridional section, of the nodoid, it is obvious 

 that the solid of revolution to be derived from it can never be 

 reahsed in its entirety : for one part of the solid figure would cut, or 

 entangle with, another. All that we can ever do, accordingly, is to 

 reahse isolated portions of the nodoid*. 



* This curve resembles the looped Elastic Curve (see Thomson and Tait, ii, 

 p. 148, fig. 7), but has its axis on the other side of the curve. The nodoid was 

 represented upside-down in the first edition of this book, a mistake into which others 

 have fallen, including no less a person than Clerk Maxwell, in his article "Capillarity " 

 in the Encycl. Brit. 9th ed. 



Fig. 109. 



